The present invention relates to a mass spectrometric method for performing tandem mass spectrometry of a sample separated by a chromatography apparatus and a mass spectrometric system suitable for carrying out the method.
Generally, in a liquid chromatography (LC) apparatus or gas chromatography (GC) apparatus-tandem mass spectrometric system, a sample to be measured is temporally separated into constituents depending on differences in times for the sample constituents to pass through the LC or GC apparatus. Subsequently, the thus separated sample constituents are ionized and a variety of created ions are sent to a mass spectrometer, in which ionic strengths are measured in respect of individual mass versus (electric) charge ratios m/z each representing a ratio of mass number m to atomicity z of an ion. A mass spectrum obtained from results of the measurement includes peaks of ionic strengths (ionic peaks), measured in respect of the individual mass versus charge ratio m/z values, of sample constituents eluted from the LC or GC apparatus at time points. The mass spectrometry of the ionized sample constituents of their own as above is called MS1. In a tandem type mass spectrometric system capable of performing a multi-stage dissociation operation, an ionic peak of an ion having a specified mass versus charge ratio m/z is selected from ionic peaks detected in the MS1 (the thus selected ionic sort is called a parent ion), the parent ion is dissociated by, in general, causing it to collide with gas molecules to create dissociated ionic sorts and the thus created dissociated ionic sorts are subjected to mass spectrometry to obtain a mass spectrum in a similar way. Here, a process for dissociating the parent ion through n stages and performing mass spectrometry of resulting dissociated ion sorts is called MSn+1. As described above, in the tandem type mass spectrometer, a parent ion is dissociated through multiple stages (first stage, second stage, . . . , n-th stage) and mass numbers of ionic sorts created in the respective stages are subjected to mass spectrometric operations (MS2, MS3, . . . , MSn+1).
(1) In most of the mass spectrometer capable of performing tandem spectrometry, the tandem spectrometry is carried out by the data dependent function in which when selecting parent ions for use in MS2 spectrometry on the basis of MS1 data obtained at individual time points of eluting from the LC or GC, parent ions are chosen in order of ionic peaks of higher strengths (for example, ionic peaks within 10 high-ranking strengths) and subjected to dissociation and mass spectrometry (MS2).
(2) One may refer to JP-A-2002-168842 and a product introduction of a system for ADME/TK, http://www.waters.co.jp/index.html which disclose known examples of a combined system of a plurality of liquid chromatography apparatus and a mass spectrometer.
The former reference is directed to a system for introducing samples to a plurality of columns by switching over valves, thus disclosing an example in which mass spectrometry is conducted by providing different samples with different elusion times in the plural columns and also disclosing an example in which mass spectrometry of the same sample is conducted by changing the separation condition the column has.
The latter reference is also directed to a tandem mass spectrometric system having a plurality of columns and a mass spectrometer in combination, disclosing a system utilizing a time-difference chromatography apparatus which starts chromatography by shifting start time.
Through the data dependent function shown in (1) as above, ions are selected in order of high-ranking strengths in MS1 data at current time regardless of MS1 data obtained before and after each elusion time and consequently, there is a possibility that either an ion of high strength kept to be eluted for a fairy long time or an ion eluted at a time point at which the ionic strength is not near a peak will be selected and ionic strength is not near a peak will be selected and subjected to MS2 spectrometry. In the former case, the same ion is taken as an object of MS2 many times for a fairy long time and in the latter case, the ionic strength deviating from the peak is responsible for reduction of strength of MS2 data itself, giving rise to the occurrence of inefficient tandem mass spectrometry.
The system disclosed in JP-A-2002-168842 intends to provide how to acquire many sorts of MS1 data, failing to presuppose any tandem mass spectrometry. The latter reference shows the tandem mass spectrometric system utilizing the time-difference chromatography apparatus which starts spectrometry by shifting start time but it does not refer to exchange of chromatogram data and mass spectrometry data among a plurality of columns, so that, in the mass spectrometer, MS2 spectrometry is conceivably carried out under the same spectrometry condition. In such a case, spectrometry can be done with high throughput but the results of MS2 spectrometry do not change with samples from the individual columns and conceivably, the efficiency of tandem spectrometry will not change.